This invention relates to packet transmission in multiple access communication systems.
In this patent document, the references referred to in square brackets, eg [1], are listed at the end of the disclosure.
The 3G CDMA system is supposed to support simultaneous voice/packet data/circuit data operations [1] with different QoS requirements. Since the traffic characteristics and the quality of service requirements for packet data services are quite different from those of voice traffic, system re-design in certain essential aspects is necessary for an integrated voice-data system without impacting voice quality or sacrificing data performance.
Packet-type services are designed for on-off sources, which generate no information during often prolonged, but unpredictable, off intervals. For many data applications the intermittent nature of the traffic is orders of magnitude greater than that of voice signals as shown in FIG. 1. The system capacity can be greatly increased if no signal is transmitted during the off period, since the CDMA system is interference limited.
The efficient provision of packet-type services is particularly difficult on reverse links (from mobile to base station) of CDMA systems, because of the need to rapidly regain synchronization of spreading sequences when the source resumes transmission after a period of silence. One possible solution to the link maintenance problem when bursty transmission is required on reverse links of CDMA systems is to assign a separate low bit rate physical control channel (created using code division multiplex) to each portable in a given cell. This approach, proposed by some investigators [2], can be called a continuous transmission medium access control scheme (CTX-MAC), and it unfortunately causes increased multi-user interference introduced by continuously transmitted maintenance signals. Increased interference naturally leads to reduced traffic capacity. Also its implementation is characterized by significantly increased complexity of base station receivers, where each physical control channel (serving one portable) would require a separate despreading correlator. If the duty cycle (ratio of the on-period to off-period of the mobile""s transmitter) as shown in FIG. 1 is small, then considerable savings in both base station hardware and system capacity may be achieved if transmission is discontinued during the off-periods and the hardware is shared among different users. This is the discontinuous transmission medium access control (DTX-MAC) scheme. With the DTX-MAC scheme, when a particular user has a data packet to send, the base station needs to be notified by the mobile of its intention to transmit, and synchronization needs to be quickly resumed. To achieve that an access request message (ARM) is transmitted from the mobile to base station to acquire synchronism, and to inform the base station of the mobile""s identity. An access reservation channel (ARC) is allocated for such a purpose. All mobiles in a cell (or sector) use the same PN code to send their ARMs on the ARC; this avoids the need to have a separate receiver for each mobile, even in the off mode. If the number of sources is large, then more than one ARC may be used.
A MAC protocol is required on the ARC for the mobiles to access the base station with individual ARMs. One possible approach is to allow the mobile units to send ARMs in an asynchronous fashionxe2x80x94ADTX-MAC protocol. Another is a synchronous approachxe2x80x94SDTX-MAC protocol, in which a slotted frame structure is used for the access control of ARMs from all users.
Since the user sending a transmission request has already been registered and has acquired synchronization before, the timing ambiguity is mainly caused by the propagation delay uncertainty due to the movement of the mobile in a cell during the silent period. Hence, it is relatively easier to synchronize again than to acquire initial synchronization during the registration phase involving the access channel. On the other hand, the synchronization needs to be regained very quickly so that the arriving data burst can be transmitted without undue delay. Due to these reasons, the specific structure of the IS-95 access channel [3] which is used by the mobile station to initiate communication with the base station and to respond to paging channel messages, is no longer suitable, since its access probe occupies several 20 ms frames in time. Instead, much shorter access request messages are used within the access reservation channel structure significantly different from the IS-95 access channel. Such an approach dramatically reduces access delay and increases throughput.
SDTX-MAC Protocol
In the SDTX-MAC system, a time-slotted frame structure is used to accommodate access requests from different users on the access reservation channel (ARC). Specific time slots in the frame equal in length to the duration of the access request message (if necessary, a small guard time may be added) are assigned by the base station to mobile users after registration (one slot per user). Collision is thus avoided and there is no need of inserting an identifier appendix after the synchronizing part of the ARM. A method of step-increase of power is used for ARMs. After each ARM, the mobile monitors the paging channel for an acknowledgment (ACK) from the base station. If the elapsed time before the ACK is received is longer than the prescribed maximum, the corresponding ARM is regarded as a failure, and next one is sent at a power level increased by a fixed step. For each user, a fixed number of ARMs of increasing powers form an ARM sequence. If the synchronization is still not acquired after the sequence of ARMs is sent, a new sequence is started, with the transmitting power starting from the lowest level. Identical ARM sequences are repeated up to a maximum allowed number of repetitions, which is assumed large enough to ensure acquisition. The synchronism acquired through the ARC may be coarse, but it is sufficiently accurate to receive data packets from the user in question on a dedicated data channel. Each data packet sent on that channel is preceded by a short synchronizing preamble to refine synchronism.
The number of slots in one slotted frame of the access reservation channel (ARC) is designed to accommodate the expected number of registered users in a cell or sector. Once a user is registered, a slot is assigned to him on the ARC, and the base station subsequently uses that assignment to identify a user sending an access request. An example is shown in FIG. 2 wherein there are four slots within one frame, Tp, TF are the slot and frame lengths, respectively, and there are three ARMs within one access message sequence.
ADTX-MAC Protocol
The asynchronous discontinuous transmission medium access control (ADTX-MAC) protocol is in general terms similar to that used in the IS-95 access channel. It is a spread slotted ALOHA [4]-[6] with p-persistence [7] (plus sequence back-off) between ARM sequences. As shown in FIG. 3 a fixed number of ARMs forms an ARM sequence (three ARMs within an ARM sequence in this figure), with the transmission power increasing consecutively by a fixed increment. There is a random back-off TR between ARMs in the sequence, if the ACK is not received before time-out, TA. If the second ARM sequence is required, there is a random back-off TS before a p-persistence test, for the next ARM sequence to start. To reduce the probability of collision, the exact transmission time of each user is pseudo-randomly delayed from the slot start time, the delay being known to the base station.
Since all the users use the same PN code to spread their ARMs, each ARM consists of one synchronization preamble and an appendix for user identification. This appendix is designed to have the same length as the preamble, and is block-coded to provide the necessary error protection. The ARM for ADTX-MAC is of twice the length of an ARM in SDTX-MAC protocol.
SDTX-MAC can provide delay characteristics superior to those of the ADTX-MAC, but ADTX-MAC""s throughput may be larger or smaller than that of the SDTX-MAC depending on the offered traffic and the selected maximum value of the randomization delay. This is also intuitive, since ADTX-MAC""s access request message (ARM) must include an appendix, which increases the access delay. On the other hand, due to PN spreading and randomization procedure, more than one ADTX-MAC""s ARM within one time slot can be received successfully without collision, and clearly only one ARM can be transmitted per slot in SDTX-MAC. Even if the time slot is normalized to enable fair comparison, ADTX-MAC may still provide better throughout characteristics. The reason is that, although SDTX-MAC does solve the collision problem, and takes advantage of TDMA structure, it does not take advantage of the interference suppression capabilities of spread spectrum, and thus its performance is not optimal. On the other hand, ADTX-MAC does take advantage of spread spectrum, but it does not solve the potential collision problem.
When the transmitted signal propagates through a multipath fading channel, its replicas will arrive at the receiving end at different times spread over the maximum excess delay interval. If we denote the maximum excess delay expressed in chip intervals by xcfx84M, it is reasonable to assume that when the temporal separation of signals arriving from different users is larger than (xcfx84M+1) chip intervals, collisions will not occur. If we assume that the start times of transmitted packets form a Poisson point process with the arrival rate xcex packets/s, and denote the duration of a packet in seconds as Tp, the offered channel traffic G (in packets) will be:
G=xcexTp
The present invention provides for an improved multiple access protocol that improves upon the ADTX-MAC protocol and the SDTX-MAC protocol.
The limitations in the SDTX-MAC and ADTX-MAC discussed above are addressed by the following invention, which has been called minislotted SDTX-MAC, or M-SDTX-MAC.
Accordingly, there is provided a method of transmitting packets in a telecommunications system. Packets are transmitted from multiple terminals, in which the packets are spread with an identical spreading sequence for each terminal. The packets are transmitted in frames. An aspect of the invention includes allocating a respective mini-slot in each frame to a particular terminal as an identification reference for a packet transmitted from the particular terminal in the respective time slot. Further, at each terminal, packets are transmitted from the terminal beginning at a time within the mini-slot allocated to that terminal, with the packet being longer than the mini-slot.
In one aspect of the invention, the method is used to gain access to packet data transmission channels in a cellular radio CDMA system. The packets may thus be access request messages (ARMs). The packets may also be data packets.
In a further aspect of the invention, there is provided a method of transmitting ARMs to gain access to a packet data telecommunications channel, comprising the steps of:
transmitting ARMs from multiple terminals as overlapping packets, where each identical packet contains several repetitions of an identical spreading sequence for each terminal; the ARMs corresponding to different terminals being arranged into frames;
allocating a respective mini-slot in each frame to a particular terminal as an initial synchronization and identification reference for an ARM transmitted from the particular terminal; each ARM being longer than its associated mini-slot, but the ARM beginning inside the mini-slot; and
at each terminal, beginning to transmit data from the terminal after a base station allocates to it a packet data transmission channel upon successful receipt of an ARM.
Each mini-slot should be sufficiently long to avoid a collision between ARMs sent from different terminals.
In a further aspect of the invention, the data packet, or ARM, is transmitted within a cell over a radio channel, the channel has a cell-size dependent propagation delay and a maximum excess delay, and each mini-slot has a length at least equal to the maximum excess delay plus twice the maximum expected propagation delay plus guard time.
The mini-slot may be allocated to a terminal upon registration of the terminal in a telecommunications system.
These and other aspects of the invention, including apparatus for implementing the invention, are described in the detailed description of the invention and claimed in the claims that follow.